Add support for balanceCheck and balanceNotify for LNURL-Withdraw

[iBobik]

When used LNURL-Withdraw wallet can also save URL for later use, so user can pull sats repetitively from remote wallet.

• https://github.com/fiatjaf/lnurl-rfc/blob/luds/14.md
• https://github.com/fiatjaf/lnurl-rfc/blob/luds/15.md

So if LNURL-Withdraw response contains balanceCheck field, it could offer to save this URL and then check it's balance on each future start. When there are some money it could offer to wihdraw them (e.g. by a banner on top of the transactions list).

As a bonus it could also implement balanceNotify, so Breez can send notification when remote wallet receives some money.

[iBobik]

Btw, I see similar notification already working in Breez for swaps:

[iBobik]

This could be useable for custodial services (e.g. receive tips to static QR code or LN address) to integrate with non-custodial wallets, so user can move funds to his wallet ASAP, but still receive donations remotely.